Polyester resin are a well known class of engineering resins from which molded articles can be fabricated. For example, heat resistant polyester resins such as polycyclohexylenedimethylene terephthalate (prepared from cyclohexanedimethanol as a diol component), as well as polyethylene naphthalate and polybutylene naphthalate (which are prepared from a naphthalenedicarboxylic acid as an acid component) are well known. Furthermore, polyester resins mainly prepared from cyclohexanedimethanol and a naphthalenedicarboxylic acid are known to exhibit high glass transition temperatures and possess excellent heat resistance properties.
However, these conventional polyester resins are either non-crystalline or, if crystalline, exhibit low crystallization rates to such an extent that complete crystallization cannot be attained. Accordingly, these conventional resins are disadvantageous in that they will gradually crystallize in service under high-temperature conditions to result in reduced physical properties. As a result, these conventional polyester resins have low dimensional stability characteristics. In addition, low crystallization rates make these conventional polyester resins unsuitable for injection molding operations (i.e., due to long mold cycle times and thus decreased productivity that results).
What has been needed therefore is a polyester resin which exhibits excellent heat resistance properties as well as a high crystallization rate so as to eliminate the above-noted problems and thereby be suitable for injection molding operations. It is towards fulfilling such a need that the present invention is directed.
Broadly, the present invention provides highly crystalline and heat-resistant copolyester resins by introducing a compatible high-crystallinity component into the molecular chain of the resin by copolymerization to thereby enhance the resin's crystallization rate. In this connection, the present invention is based on the discovery that a copolyester prepared from cyclohexanedimethanol as a diol monomer component and an acid monomer component comprising both (A) a naphthalenedicarboxylic acid, and (B) 4,4'-diphenyldicarboxylic acid (as a high-crystallinity component) at a specific ratio exhibits enhanced crystallization rates as well as high heat-resistance properties.